Beta adrenergic agonists (β-agonists), or phenethanolamines, are registered around the world for use in targeted livestock species to improve lean tissue deposition and feed efficiency. Two β-agonists are approved for use in ruminant animals (specifically cattle) in targeted markets (United States, Mexico, Canada and South Africa). The first is Ractopamine hydrochloride (RAC, Optaflexx™, Elanco Animal Health, Greenfield, Ind.) registered to improve growth performance, feed efficiency and carcass characteristics, while the second Zilpaterol hydrochloride (Zilmax™, Merck Animal Health) is approved for use during the finisher period to increase live weight gain and improve feed efficiency of cattle. Although both compounds are β-agonists, the products act with differing affinity on β-receptors. Both products are cleared from the animal rapidly, reflected in the NIL or very short (12 hr) withhold restrictions across animal species and in a range of markets. In addition to the product registrations for use in cattle, several research papers have observed similar improvements in growth performance and feed efficiency with RAC or Zilpaterol supplementation in lambs at constant or step up inclusion rates between 10-30 ppm. The label recommendations for Optaflexx™ suggest a dose of 11-27 ppm for growth; feed efficiency and carcass leanness improvements in cattle. The majority of studies investigating the effects of RAC on lamb performance utilise concentrations around 20 ppm (20 g/t). Given Zilpaterol is not approved for use in Australia in any animal species, the following investigation focused on ractopamine.
One of the issues with the use of β-agonists for improving growth performance and feed efficiency is that the response diminishes over time due to the down regulation of β-receptors. It would be advantageous if the response to β-agonists for improving growth performance and feed efficiency could be prolonged.
The present inventors have developed a method for prolonging the feed efficiency benefits of ractopamine and improving body composition by increasing lean tissue and reducing adipose tissue deposition in ruminants.